Task chairs or office-type chairs have evolved over time to improve the support provided to chair occupants and to provide chairs that better meet the usage needs of the occupant. One improvement made in chairs, and more particularly the tilt mechanisms of chairs, is the synchronization of the back and seat to one another. On these mechanisms, the back and seat are synchronized so that as the back reclines, the seat moves as well. These synchronized mechanisms are also referred to as “synchrotilt” mechanisms. These mechanisms are usually designed so that the back moves at a greater rate than the seat.
When designing a sychrotilt mechanism, a designer considers several factors. One factor is the “shirt shear” resulting from the design. Another factor is the “bridging” resulting from the design. Shirt shear is the tendency of the chair movement to pull and/or push an occupant's shirt during recline. If shirt shear occurs, the occupant's shirt is untucked, which is undesirable. Bridging is the condition experienced when the lower back support falls away from the occupant during recline. When bridging occurs, the occupant's lumbar area is largely unsupported by the chair back. But it is desirable to provide the occupant proper lumbar support throughout the range of motion of the chair, such that the occupant has lumbar support in the upright position, the reclined position and any position in-between. The designer thus strives to minimize shirt shear and bridging.
Another factor considered by the synchrotilt-mechanism designer is the effect recline has on the occupant's center of gravity over the chair base. It is desirable to allow an occupant to fully recline while not moving the occupant's center of gravity so much that an over-balancing or tipping condition occurs. This provides the occupant with an increased sense of comfort while in the chair. Moreover, if the occupant's center of gravity is maintained centrally over the base assembly of the chair, a smaller base assembly can be used. The chair designer is thus offered increased design flexibility in choosing a base assembly.
An additional design factor is the position of the occupant's eyes relative to the work surface while the occupant is in various positions in the chair. This design factor can be called the “viewing distance” factor. It is desirable to alter the viewing distance as little as possible as the occupant moves from an upright position to a fully-reclined position. This allows the occupant's eyes to remain approximately the same distance from a working surface in either the upright position or reclined position, without further manipulation of the chair position. For example, if an occupant is operating at a computer terminal, it is desirable to maintain the distance of the occupant's eyes relative to the computer display from the upright position through the fully-reclined position. If this is achieved, the occupant is not required to move the chair forwardly after reclining to adjust for a change in viewing distance. Similarly, it is also desirable to maintain the position of the occupant's arms and hands as much as possible from the upright position to the reclined position. If this is achieved, the occupant can continue working, such as at a keyboard, whether in the fully-upright position, the fully-reclined position, or any intermediate position.
Yet another factor considered by chair-mechanism designers is the reclining “dwell” of the chair. On most chair mechanisms, the back or seat of the chair is biased so that the chair will easily return to the upright position when the reclining force is relieved. In these chairs, the initial force needed to recline the chair is greater than the force needed to maintain the chair in the reclined position. The “dwell” is the force needed to maintain the chair in the reclined position. It is most desirable to design a mechanism easily adjusted so that the dwell force approximates the force applied by the weight of the occupant's upper body. If this is achieved, the occupant can maintain the chair in a variety of reclined positions with very little effort.
In addition, the biasing mechanism used to return the chair to the upright position from a reclined position is adjustable on many chairs. The adjustment mechanism is provided so that once adjusted a range of occupants having a range of body types can use the chair comfortably. But the biasing mechanisms are typically large springs that are difficult to adjust properly. This results in occupants using the chair in a condition that is not optimized for their particular body type. It would be desirable to provide a synchrotilt mechanism that did not rely on a large-spring-biasing mechanism to bias the chair to the upright position and that accommodated a large range of occupant body types without significant adjustment.
It would be desirable, therefore, to provide a synchrotilt mechanism for use on a chair that optimally satisfies the above design considerations and overcomes the existing drawbacks and deficiencies of prior art chairs. A synchrotilt mechanism is needed that minimizes bridging and shirt pull so that an occupant's back is properly supported during recline of the chair. Moreover, a synchrotilt mechanism is needed that maintains the viewing and reach distance while the occupant is in a reclined position in the chair and that allows the occupant to recline while maintaining the center of gravity generally over the base of the chair. Finally, a synchrotilt mechanism is needed that balances the recline dwell of the chair in a way that accommodates a wide range of occupants without significant adjustments to the chair or mechanism.